This invention relates generally to an improved regulating system for stabilizing the operation of a current fed induction motor drive system and, more particularly, to a system for stabilizing motor operation during the transition from a motoring or propulsion mode to an electrical braking mode.
An induction motor typically comprises a squirrel cage or wound rotor that is mounted in a stator having windings connected to a suitable source of excitation. Excitation of the stator windings creates a magnetic flux across the stator-rotor air-gap of the motor and the current induced in the rotor interacts with the air-gap flux to produce an electromagnetic force or torque tending to move the rotor relative to the stator. The amount of torque developed by the motor is often expressed in terms of the magnitude of the air gap flux and the slip frequency between stator and rotor. The effective slip frequency by definition is the difference between the frequency of the flux wave on the air gap and the equivalent electrical frequency at which the motor shaft is rotating (i.e., motor speed). Where such a motor is required to run at variable speeds with variable loads and in both forward and reverse directions, as in the case of traction motors for electrically propelled vehicles, the stator windings are advantageously supplied with polyphase a-c power which is so conditioned that the frequency as well as the amplitude of the stator excitation are adjustable as desired and the phase sequence is reversible.
A review of control systems for current fed induction motor drives is provided in U.S. Pat. No. 4,088,934 issued for J. D. D'Atre, T. A. Lipo, and A. B. Plunkett and assigned to General Electric Company. That patent teaches stabilizing a current fed induction motor drive system by controlling the excitation source of the motor as a function of the actual phase angle between the air gap flux and the stator current in the motor. The control system varies the frequency of the current supplied to the stator so as to regulate the phase angle to a desired value. In addition, the magnitude of air-gap flux is directly monitored and the magnitude of excitation applied to the stator is controlled so as to regulate the magnitude of air-gap flux to a desired value.
When the aforementioned system is operated in an electrical braking mode, it has been found that the system exhibits more stable characteristics if the magnitude of stator current rather than the magnitude of air-gap flux is regulated to a desired value. However, in the transition region, typically at slip frequencies between plus and minus three Hertz, the slope of the torque versus slip characteristic for any particular regulated current magnitude is relatively steep. In other words, torque magnitude may vary from zero to its maximum value with only a 3 Hz variation in slip frequency. Furthermore, the motor characteristic for constant current operation is a two-value function, i.e., for a given magnitude of stator current, two different distinct values of slip frequency will result in the same torque output. Satisfaction of the desired torque output in the transition region is difficult since very small slip frequency variations result in very large torque variations. Accordingly, operation at less than 3 Hz may be unstable and result in undesirable torque oscillations.